The present invention relates generally to constant current driver circuits. More particularly, this invention pertains to constant current direct current (DC) driver circuits for providing power for light emitting diode (LED) light sources.
LED driver circuits limit the maximum voltage developed across an LED load by reducing the driven current. Referring to prior art FIG. 1, a maximum operating voltage, “Vnominal” 102, and maximum operating current, “Ibright” 104, are defined. If the magnitude of the current reaches a shutdown current threshold, “Ishutdown” 106, or a shutdown voltage threshold, “Vshutdown” 108, the driver circuit shuts down to protect itself. A driver circuit with a fast control loop attempting to start an abnormally high impedance load (i.e., light source such as an LED) will likely shut down. Shutting down commonly requires cycling of a power source powering the driver circuit (e.g., turning a light switch off and back on) to restart the driver circuit. Not all driver circuits are designed to shut down when unloaded (i.e., when the voltage is above the nominal or maximum operating voltage 102 and even above the shutdown voltage 108) and will, therefore, maintain a high and unsafe output voltage. When an LED load is reattached to a driver circuit that is generating an abnormally high output voltage, the LED load experiences high surge currents which can instantly and permanently damage the LEDs. While these behaviors protect the driver circuit from excessive component stress and damage, they can damage the load (e.g., LED light source), and the behavior of the driver circuit is not defined for a voltage below a minimum operating voltage, “Vmin” 110, or a voltage below a minimum operating current, “Imin” 112.
Driver circuits are designed to safely function within the intended range of operation indicated on FIG. 1 as Nominal Operation 120 bounded by the minimum and maximum operating voltages and minimum and maximum operating currents. Functioning outside of nominal operation can damage power transfer components of the driver circuit or cause the load to operate in an unstable manner. One common approach to deal with possibly operating outside of Nominal Operation 120 (i.e., below the minimum operating current 112 and/or below the minimum operating voltage 110) is to shut down the driver circuit and cease to provide power to the load (i.e., the LED light source ceases to provide light).